1. Field of the Invention
This invention relates to a semiconductor memory device and a manufacturing process for the same. More particularly, this invention relates to a non-volatile or large-capacitance semiconductor memory device in which a thin film of a dielectric material having a high dielectric constant or of a ferroelectric material is used as a capacitive insulation film, and a manufacturing process for the same.
2. Description of the Related Art
Ferroelectric films have features such as self polarization and high dielectric constant characteristics, and the ferroelectric films have hysteresis characteristics between polarization and electric fields of ferroelectric substances. Accordingly, as non-volatile memories, semiconductor devices making use of ferroelectric films are on research and development. Also, the ferroelectric films have much larger dielectric constant than silicon oxide films so that memory cells can be made to have smaller area when the ferroelectric films are used as capacity insulation films. Hence, they can be used as large-capacitance highly integrated RAMs (random access memories).
Semiconductor devices making use of ferroelectric films are commonly made up by forming a capacitor layer of the ferroelectric film usually on an MOS transistor and thereafter further forming a top wiring layer, an insulation layer and a passivation layer.
The ferroelectric films are comprised of a sintered body of a metal oxide, and contain oxygen which is rich in reactivity. Conventional semiconductor memory devices in which capacitors are formed by using such ferroelectric films in the capacity insulation films are so constructed that noble-metal electrodes which are stable to oxidation reaction, as exemplified by those chiefly composed of platinum, are formed in the upper and lower electrodes of the capacity insulation films to form the capacitors. Then, after the capacitors have been formed, interlayer insulation films and devices are formed, and thereafter passivation films are formed on the outermost surfaces of the devices.
As structures concerning the above ferroelectric films, proposals are disclosed in Japanese Patent Applications Laid-open No. 7-273297, No. 11-126883, No. 2000-164817 and so forth.
In the above conventional semiconductor memory devices, silicon nitride or silicon oxide is used as the interlayer insulation films and passivation films. Such interlayer insulation films and passivation films are usually formed by CVD (chemical vapor deposition) and hence hydrogen generated from material gases is often incorporated in the films. Also, the silicon nitride can prevent water from entering, but does not have any barrier properties against the hydrogen.
Meanwhile, ferroelectric films are known to undergo deterioration of polarization characteristics upon their annealing in an atmosphere of hydrogen. This deterioration is presumed to be caused by the platinum of upper and lower electrodes which reacts with hydrogen to act as a reducing catalyst to reduce the ferroelectric film.
Although the ferroelectric films have the characteristic of being reduced with ease, the above conventional semiconductor memory devices make use of materials which contain hydrogen, in the interlayer insulation films and passivation films of the devices. Hence, they have a problem of causing deterioration of electrical properties of ferroelectric devices.
It is also known that the ferroelectric films have piezoelectric properties and hence are used in piezoelectric devices. Such films are susceptible to inner stress of the devices. This also is a factor to cause deterioration of electrical properties of ferroelectric devices. Accordingly, it is important that the interlayer insulation films and passivation films used in semiconductor memory devices are formed using materials which do not contain any hydrogen and are films made to have less stress.